<?
function resize($img, $w, $h, $newfilename) {

//Check if GD extension is loaded
if (!extension_loaded('gd') && !extension_loaded('gd2')) {
trigger_error("GD is not loaded", E_USER_WARNING);
return false;
}

//Get Image size info
$imgInfo = getimagesize($img);
switch ($imgInfo[2]) {
case 1: $im = imagecreatefromgif($img); break;
case 2: $im = imagecreatefromjpeg($img);  break;
case 3: $im = imagecreatefrompng($img); break;
default:  trigger_error('Unsupported filetype!', E_USER_WARNING);  break;
}

//If image dimension is smaller, do not resize
if ($imgInfo[0] <= $w && $imgInfo[1] <= $h) {
$nHeight = $imgInfo[1];
$nWidth = $imgInfo[0];
}else{
			  //yeah, resize it, but keep it proportional
if ($w/$imgInfo[0] > $h/$imgInfo[1]) {
 $nWidth = $w;
 $nHeight = $imgInfo[1]*($w/$imgInfo[0]);
}else{
 $nWidth = $imgInfo[0]*($h/$imgInfo[1]);
 $nHeight = $h;
}
}
$nWidth = round($nWidth);
$nHeight = round($nHeight);

$newImg = imagecreatetruecolor($nWidth, $nHeight);

/* Check if this image is PNG or GIF, then set if Transparent*/  
if(($imgInfo[2] == 1) OR ($imgInfo[2]==3)){
imagealphablending($newImg, false);
imagesavealpha($newImg,true);
$transparent = imagecolorallocatealpha($newImg, 255, 255, 255, 127);
imagefilledrectangle($newImg, 0, 0, $nWidth, $nHeight, $transparent);
}
imagecopyresampled($newImg, $im, 0, 0, 0, 0, $nWidth, $nHeight, $imgInfo[0], $imgInfo[1]);

//Generate the file, and rename it to $newfilename
switch ($imgInfo[2]) {
case 1: imagegif($newImg,$newfilename); break;
case 2: imagejpeg($newImg,$newfilename);  break;
case 3: imagepng($newImg,$newfilename); break;
default:  trigger_error('Failed resize image!', E_USER_WARNING);  break;
}
 
 return $newfilename;
}
/*-----
Test image
------*/

$img = "some/dir/image.png"; // File image location
$newfilename = "thumb_image.png"; // New file name for thumb
$w = 100;
$h = 100;
 
$thumbnail = resize($img, $w, $h, $newfilename);
 
echo "<img src='".$thumbnail."'>";

?>